Description

Adjusts the contrast of an image with a non-linear sigmoidal contrast
algorithm. Increase the contrast of the image using a sigmoidal transfer
function without saturating highlights or shadows. Contrast indicates
how much to increase the contrast (0 is none; 3 is typical; 20 is
pushing it); mid-point indicates where midtones fall in the resultant
image (0 is white; 50 is middle-gray; 100 is black). Set sharpen to
TRUE to increase the image contrast otherwise the contrast is reduced.

Errors/Exceptions

User Contributed Notes 2 notes

The documentation for this function really isn't useful or helpful at all. "Sharpen", "Contrast" and "Midpoint" aren't the names of the params, which are referred to as "alpha" and "beta".

The Imagemagick manual entry:

For those interested, the corrected formula for the 'sigmoidal non-linearity contrast control' is...( 1/(1+exp(β*(α-u))) - 1/(1+exp(β)) ) / ( 1/(1+exp(β*(α-1))) - 1/(1+exp(β*α)) )Where α is the threshold level, and β the contrast factor to be applied.The formula is actually very simple exponential curve, with the bulk of the above formula is designed to ensure that 0 remains zero and 1 remains one. That is, the graph always goes though the points 0,0 and 1,1. And the highest gradient of change is at the given threshold.